This invention relates to a method of detection of objects which emit respectively unique supervisory signals that can collide, within range of one or more receivers.
It is necessary in some environments to detect the presence of objects which are within certain regions. For example, in a hospital it is necessary to determine general locations of respective professional staff and/or patients; at a convention it may be necessary to locate the general locations of attendees; in a laboratory it may be necessary to locate portable equipment; in a city it may be necessary to locate the general locations of police, or of particular automobiles, the presence and/or location of a person under house arrest may need to be determined, etc.
Various systems exist in which transmitters are attached to the object to be located, e.g. via tags. The transmitters transmit from time to time, which transmissions are received by various receivers. The various receivers detect the transmissions and report the presence of the object within their respective reception ranges. A typical transmission is comprised of an identification (ID) of an object (or of the tag which is attached to the object). If an ID is not received, it is either not within range of the receiver, or if previously detected to be within range, an alarm can be raised.
However, collisions between transmissions from various objects is a problem which results in faulty reception, and which can cause alarms to be raised simply because a collision between properly transmitted ID signals resulted in no reception of some or all of the collided ID signals. For that reason, various schemes have been used to try to avoid the collisions.
For example, in U.S. Pat. No. 5,686,902 a tag location system is described in which the tags respond to interrogation signals. The problem of collisions is addressed using two solutions:
(a) The tag response time is made short relative to the ID collection (listen) time of the receiver. When a tag responds to an interrogation signal, the interrogator transmits directed acknowledgement signals to the tags which shuts off the tags whose Ids have been successfully received by the interrogator (i.e. those whose responses have not collided). This reduces the number of tags left to respond, thus reducing the likelihood of collisions.
(b) The interrogator listen period is a function of the number of tags which respond (i.e. the listen period equals the number of tags multiplied by the response time of a tag after receipt of an interrogation signal). The listen time is therefore reduced when tags whose IDs have been successfully received are shut off, and the number of responding tags thereby reduced.
If the ID of a tag has not been received during the listen time, an alarm is raised.
However, this is not suitable for systems in which the tags must be inexpensive, for example throwaway items. It is also not suitable in which the tags must merely transmit their Ids randomly, without interrogation. The patented system requires each tag to include a radio receiver and logic circuits which can detect an interrogation signal, to enable a response, to detect an addressed acknowledgement signal and to shut off, and to further contain circuits which can wake up the tag receiver to listen to subsequent interrogation signals. This is expensive, and the tags are unlikely to be used in a tag throwaway system.
In U.S. Pat. No. 5,539,394 a system is described in which transmissions from the tags are synchronized, tags emitting signals in predetermined time slots following reception of a xe2x80x9cstartxe2x80x9d signal. The number of tags is in excess of the number of time slots, and therefore collisions are expected to occur. In the case of collisions occurring due to several tags transmitting in the same time slot, a base (hash) number on which the time slots of the colliding tags is based is changed, by downloading. Acknowledgement signals are sent to tags which have already been detected, to cause them to stop transmitting. Therefore as the number of tags is reduced, there will be fewer, or no collisions occurring.
The latter system has similar problems as the former, in that the tag must contain, besides an ID transmitter, a receiver with circuitry to receive the hash number and the acknowledgement signals, to stop transmitting, and to wake up at a particular time. This circuitry is expensive and would be unlikely to be used in a tag throwaway system.
In general the latter system reduces the number of tags transmitting in a fixed listen time.
In the description herein, the term xe2x80x9ctagsxe2x80x9d will be used synonymously with xe2x80x9cobjectsxe2x80x9d; on the basis that if the object itself does not include or consists of transmission circuitry, a tag which does will be attached to a non-transmitting object, converting it to a transmitting object.
The present invention uses tags which preferably randomly transmit supervisory messages which preferably contain the respective IDs of the tags, do not require receivers, and therefore its cost can be reduced.
With the tags transmitting randomly, with no required interrogation or requirement for a shut-off command to be received, a receiver in the present invention can determine the presence of each tag in its reception region by increasing its xe2x80x9clistenxe2x80x9d interval to a point at which no collisions are detected (to some arbitrary limit). The IDs of the detected tags can be retained in a table (or the IDs of expected tags can be received from another device such as a remote computer) and retained in a table.
In accordance with an embodiment of the present invention, a method of detecting a variable number of objects within range of a receiver comprises:
(a) transmitting from each object a supervisory message from time to time,
(b) detecting the supervisory message by the receiver over a detection interval, and
(c) varying the detection interval based on a number of objects under supervision and the probability of collisions of the supervisory messages.
In accordance with another embodiment, the variation of the detection interval is undertaken by increasing it to the extent that no collisions of any previously undetected supervisory message occurs.